Features

James Umen, Ph.D.

Algae are hidden drivers of global ecosystem productivity with unrealized potential. We seek to unlock the secrets of algal growth, reproduction and development in an environment where basic discoveries can be transformed into real-world innovations in biofuels, agriculture and medicine.

Research Summary

Using algae as a primary experimental system, Jim’s research is investigating cell size control, cell growth regulation and carbon partitioning, and evolution of multicellularity.

I maintain a diverse and interdisciplinary research program aimed at answering fundamental questions in eukaryotic cell biology, evolution and development using the green algae Chlamydomonas reinhardtii (Chlamydomonas) and Volvox carteri (Volvox), as well as some of their close relatives in the volvocine algal family. Green algae are the smallest and simplest members of the green plant lineage, and are an ecologically important group of organisms for their role in the global carbon cycle. They are also excellent models for many areas of basic plant biology and for eukaryotic cell biology. Our research takes advantage of unique aspects of Chlamydomonas and Volvox to answer questions that can ultimately impact agriculture and human health. The most direct translational impacts of our work will be in algal biotechnology where cell size, growth metabolism, and oil composition are important yield traits, while our work on sexual cycles may enable development of breeding and improvement strategies for algal crop species.

Our work is in three main areas: 1. Cell size homeostasis and cell cycle control; 2. Cell growth control and carbon metabolism; and 3. Evolution of sexual dimorphism and germ-soma differentiation. The three topics are described separately below, but there are technical and conceptual connections and synergies between them that we have exploited to advance our work in all three areas.
Cell Size Homeostasis and the RB Tumor Suppressor PathwaySize homeostasis is a fundamental property of proliferating cells and is thought to be governed by cell size checkpoints. The multiple fission cell cycle of Chlamydomonas uncouples cell growth and division and allows us unique access to a size checkpoint mechanism. A key regulator of this checkpoint is the Chlamydomonas retinoblastoma (RB) tumor suppressor pathway, whose function in cell size and cell cycle regulation is a major focus of investigation.

Cell Growth Regulation in Photosynthetic Eukaryotes
Cell growth in eukaryotes requires the coordinate regulation of cytoplasmic biosynthetic processes with those in chloroplasts and mitochondria, semi-autonomous organelles that contain their own protein biosynthetic machinery. Chloroplasts from higher plants and green algae represent a large fraction of cellular biomass yet it is unknown how their growth is regulated with respect to cytoplasmic growth. The TOR (target of rapamycin) kinase signaling pathway is conserved in all eukaryotes where it functions as a nutrient-sensitive modulator of growth rates. We are using Chlamydomonas as a simple model for how TOR signaling contributes to coordinated growth control in photosynthetic eukaryotes.

Evolution of Developmental ComplexityChlamydomonas reinhardtii belongs to a diverse clade of green algae, some of which have undergone a remarkable transition to multicellularity. The best-characterized of the multicellular relatives is Volvox carteri, a species that embodies many of the hallmarks of multicellular metazoans or plants. These include terminally differentiated somatic cells, reproductive stem cells, complex embryonic patterning, and formation of sexually dimorphic germ cells (eggs and sperm), none of which are present in its unicellular relative Chlamydomonas. Indirect evidence suggests that the RB tumor suppressor pathway might be coupled to germ/soma differentiation and to dimorphic germ cell production. Our current work is aimed at cloning and characterizing the mating locus that is the genetic determinant for sperm and egg formation.